384 CHEMICAL ENGINEERING.
and finished in a Carlson evaporator, utilizing the waste heat from the rotary furnaces. Evaporators must have cast-iron or steel shells and wrought-iron tubes. Precautions against foaming are very necessary.
Evaporators for the recovery of soda or sulphate pulp liquors must be washed frequently in order to remove the scale which is formed by the organic matter in the liquor.
Sulphite Waste Liquor.—One ton of pulp will produce from 2,500 to 5,000 gal. of waste liquors, the density of which will vary from 6 to 10 per cent. It is usually concentrated after neutralization to about 30°B6. (50 per cent solids) in multiple-effect evaporators of the vertical or rapid-circulation type, and the capacity is from 2% to 3K gal. per square foot, with a steam pressure of 5 Ib. and a vacuum of from 27 to 28 in. The tubes will foul very quickly, on account of the large amount of calcium sulphate contained in the weak solution, and therefore evaporators are to be constructed so that tubes can readily be cleaned mechanically.
Evaporators are usually built with cast-iron or steel shells and with wrought-iron tubes. Sometimes, where the liquor is not quite neutralized, copper tubes are to be preferred. In special cases where the waste liquor is not neutralized at all, all copper construction is necessary, which may be replaced by evaporator shells of cast-iron or steel with acid-proof brick lining and copper tubes.
Mercerizing Liquors.—Quantities of liquor will vary considerably with the process of washing used in each plant, and a reasonable average will give 2 gal. of 6°Twaddell waste liquor for each pound of cotton mercerized. These liquors are usually concentrated to from 70 to 80°Twaddell in a triple- or quadruple-effect evaporator of the horizontal-tube or rapid-circulation type. The capacity is from 2H to 3K gal. per square foot, with a steam pressure of from 10 to 15 Ib. Special construction and entrainment separators are necessary to prevent losses by foaming. The last effect will frequently show incrustations of sodium carbonate which can be dissolved in hot water or weak liquor. Evaporators are usually built of steel with steel or charcoal-iron tubes.
Wool-scouring Waste.—According to the method of treatment, the quantities of waste liquor recovered will vary quite a great deal, and 1,000 Ib. of wool will give from 1,000 to 1,500 Ib. of waste liquors containing about 5 per cent solids. They are concentrated to 60 per cent in a double- or triple-effect evaporator of the horizontal-tube or rapid-circulation type, and the capacity is from 2 to 3 gal. per square foot with a steam pressure of 5 Ib. and a vacuum of 26 to 27 in. In some cases, the liquor is further concentrated to about 75 per cent with live steam pressure of from 30 to 50 Ib., and at a capacity of about K gal. per square foot. The liquors are usually difficult to handle on account of excessive foaming, and tubes are apt to foul on account of incrustations consisting mostly of organic matter. Evaporators are built of steel with wrought-iron or steel tubes, as the liquors are strongly alkaline.
Distillery Slop.—One bushel of corn or rye will give from 25 to 30 gal. of liquor containing from 1H to 2 per cent solids. They are concentrated to about 35 per cent solids usually in countercurrent, and the capacity is from 2% to 3 gal. per square foot with a steam pressure of 5 Ib. and a vacuum of 27 in. Usually a triple- or quadruple-effect is used and good results have been obtained by reversing the current of the liquor through the evaporators; otherwise tubes will foul